You probably have heard of this expression before if you are a runner and may have even experienced it yourself. ¨Hitting the wall¨ usually happens around the 29th to 35th kilometer. The runner´s pace slows down considerably, the legs become very heavy and thinking often becomes hard and confused (now that I think about it, this last thing happens to me quite often). This happens because we basically run out of available energy.
The runner´s primary sources of energy during prolonged exercise are carbohydrates and fats. We have lots of stores of fat, around 70.000 to 75.000 kcla, even in a lean adult, but the fat metabolism requires a constant supply of oxygen, and delivery of energy is slower than that provided by the carbohydrate metabolism. The carbs reserves (glycogen), on the other hand, are quite limited and have only around 2.000 to 2.400 kcla, which happens to be enough energy to get us to kilometer 29-30. Since the body is much less efficient at converting fat to energy, running pace slows and the runner suffers from fatigue.To make things even worse the brain, which only accounts for 2% of your weight and consumes 20% of your energy, gets its fuel source only from carbohydrates!! So now next you ¨hit the wall¨you will know why that is. Hope you liked it. Till next time
Kenney L, Wilmore J, Costill D. Physiology of Sport and Exercise sixth edition, Human Kinetics,2015.